Switching patients to providers who perform a minimum number of surgeries is gaining popularity as a way to reduce operative deaths. But the independent effects of surgeon versus hospital volumes are unclear: is an operation safer in the hands of a high-volume surgeon in a low-volume hospital versus a low-volume surgeon in a high-volume hospital? Does the "safe region" rely differently on surgeon versus hospital volumes for different operations? Hierarchical models have recently been used to estimate the independent effects of surgeon and hospital volumes on surgical outcomes. However, they may not have adequately accounted for the inherent confounding of these factors. We will address this issue using a novel methodological modification to reexamine the data from an important recent study on surgery volume standards for hospitals and surgeons. The modification is intuitive. For example, we examine mortality as a function of surgeon volume only via comparisons with surgeons operating in the same hospital. Formally, we will use a hierarchical model with hospital-level fixed effects. Preliminary application of this approach for two cardiovascular procedures ~ coronary-artery bypass grafting and elective repair of an abdominal aortic aneurysm, indicate that correcting for this confounding leads to significantly different findings. We have permission to use a large, analytic Medicare patient-level outcomes file for 14 high-risk cardiovascular procedures and cancer resections. These data enable rapid and cost-effective completion of the current study. For each procedure we will report: i) findings relating to minimum volume standards for surgeons and hospitals, and ii) estimated reductions in operative deaths that could be achieved by imposing appropriately tailored minimum volume standards for hospitals and/or surgeons. Using the results of the proposed study as a guide, our long-term objective is to explore key process differences that underlie operative outcomes.